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Crystallographic data and magnetic properties of RT6Ge6 compounds (R Sc, Y, Nd, Sm, GdLu; TMn, Fe)
Journal of Alloys a n d Compounds, 185 (1992) 99-107 JALCOM 222
99
Crystallographic data and magnetic properties of RT6Ge6 compounds (R-Sc, Y, Nd, Sm, Gd-Lu; T-Mn, Fe) G. Venturini, R. W e l t e r a n d B. M a l a m a n Labovatoire de C h i m i e du Solide Mindral, Associd a u CNRS No. 158, Universitd de Nancy, I, B.P. 239, 54506, Vandoeuvre les N a n c y Cedex (France)
(Received December 12, 1991)
Abstract The iron and manganese germanides RT6Ge6 (R-=Sc, Y, Nd, Sin, Gd-Lu; T=--Mn, Fe) have been investigated by X-ray diffraction and susceptibility measurements in the temperature range 90-900 K. They crystallize in the known HfFe6Ge6, YCo6G%, TbF%Sn6 or HoFe6Sn 6 type of structure except GdFe6Ge6 which is of a new type. All the compounds are anti-ferromagnetic except the RMn6Ge6 (RmNd, Sm, Gd) compounds which behave ferrimagnetically. Weak coercive fields (down to 1 kOe) are measured in RMn6G% (R~Nd, Sm).
1. I n t r o d u c t i o n P r e v i o u s s t u d i e s o n RMn6Sn8 c o m p o u n d s h a v e s h o w n the i n t e r e s t i n g m a g n e t i c p r o p e r t i e s o f this c l a s s o f m a t e r i a l [1, 2]. S o m e of t h e m exhibit large c o e r c i v e fields b u t the o r d e r i n g t e m p e r a t u r e of t h e s e f e r r i m a g n e t i c c o m p o u n d s r e m a i n s r a t h e r w e a k . In a n o t h e r way, a c r y s t a l l o g r a p h i c s t u d y o n RFe6Sn6 [3] c o m p o u n d s a l l o w e d u s t o p o i n t o u t the o c c u r r e n c e o f a l o n g - r a n g e o r d e r i n g in t h e s e c o m p o u n d s w h i c h w a s p r e v i o u s l y r e p o r t e d as i s o t y p i c of d i s o r d e r e d YCo~Ge6-type s t r u c t u r e [4]. Up to now, t h e c o r r e s p o n d i n g m a n g a n e s e a n d iron g e r m a n i d e s r e s p e c t i v e l y h a v e n o t b e e n or h a v e b e e n partially s t u d i e d [5, 6]. In o r d e r to e n h a n c e t h e t r a n s i t i o n t e m p e r a t u r e s , we h a v e i n v e s t i g a t e d t h e s e s y s t e m s .
2. E x p e r i m e n t a l d e t a i l s The compounds were synthesized from stoichiometric amounts of the e l e m e n t s . T h e m i x t u r e w a s c o m p a c t e d in a steel die a n d s e a l e d in a silica t u b e u n d e r a r g o n . T h e y w e r e t h e n h e a t e d f o r 2 w e e k s at 1 1 7 3 K for t h e iron c o m p o u n d s a n d 1073 K f o r t h e m a n g a n e s e c o m p o u n d s , e x c e p t f o r s a m a r i u m a n d n e o d y m i u m c o m p o u n d s w h i c h w e r e a n n e a l e d at l o w e r t e m p e r a t u r e ( 9 7 3 K). T h e p u r i t y o f t h e s a m p l e s w a s c h e c k e d b y X-ray analysis (Guinier Cu Ka, a n d c u r v e d d e t e c t o r I N E L CPS 120 Co Ker). T h e p a r a m e t e r s
0925-8388/92/$5.00
© 1992- Elsevier Sequoia. All rights reserved
100
were refined by a least-squares procedure. Magnetic measurements were carried out on a Faraday balance in the temperature range 8 0 - 8 5 0 K and under fields up to 1.5 T.
3. R e s u l t s
3.1. Crystallographic data 3.1.1. Manganese compounds Twelve new germanides have been characterized: E = S c , Y, Nd, Sm, Gd-Lu. The crystallographic features of these compounds are summarized in Table 1. Scandium, yttrium and the heavier rare earth element compounds crystallize in the HfFe~Ge~-type structure [7] while neodymium and samarium (i.e. the larger rare earth elements) compounds crystallize in the YCo6Ge6 structure. It is worth noting that the series of the manganese germanides is longer than that of manganese stannides since NdMnBGe6 is stable whereas NdMnsSn6 does not exist. Such a result is apparently surprising since generally the replacement of germanium with tin shifts the stability range of a given structural type towards the larger rare earth elements. This behaviour is probably due to close Sn-Sn contacts as we have previously remarked in the refinements of ScMn6Sn6 and TbMn6Sn6 [8]. According to the variation of the cell volumes, in YbMn6GeB, ytterbium seems to be in the III+ valence state.
3.1.2. Iron compounds Ten iron germanides have been characterized: R-=Sc, Y, Gd-Lu. The cell parameters, space groups and structure type of these compounds are summarized in Table 2. The YbFesGe6 cell does not exhibit any volume anomaly. The smaller rare earth elements (scandium, ytterbium, lutetium) TABLE 1 Cell parameters and structure types of RMn6Ge8 (R------Sc,Y, Nd, Sm, Gd-Lu) a Compound
a (.~)
c (~)
V (/~3)
Type
ScMn6Ge8 YMI~Ge6 NdMn6Ge8 SmMn6Ge~ GdMn6Ge 6 TbMn6Ge6 DyMn6Ge6 HoMn~Ge8 ErMn~Ge8 TmMn~Ge6 YbMn~Ge6 LuMnsGe8
5.177(6) 5.233(2) 5.258(1) 5.245(2) 5.242(2) 5.232(1) 5.228(1) 5.230(1) 5.221 (2) 5.219(1) 5.212(1) 5.211 (1)
8.084(4) 8.153(3) 8.208(1) 8.187(2) 8.184(3) 8.169(2) 8.163(2) 8.155(2) 8.142(3) 8.139(1) 8.132(1) 8.134(3)
187.6 193.4 196.5 195.0 194.8 193.7 193.2 193.2 192.2 191.9 191.3 191.3
HfFesGe8 HfFe6Ge8 YCo6Ge8 YCosGe8 HfFe6Ge6 HfFesGe8 HfFesGe8 HfFesGe8 HfFe~Ge6 HfFe~Ge8 HfFesGe8 HfFe6Ges
"Space group, P 6 / m m m .
101 TABLE 2 Cell parameters and structure types of RFe6Ge6 (R-=Sc, Y, Gd-Lu) Compound
a (/~)
b (/~)
c (/~)
ScF%G% YF%Ge6 GdFe6Ge6 TbFe6Ge6 DyF%Ge~ HoFe6Ge6 ErF%G% TmFe6G% YbFe6Ge6 LuFe6G%
5.087(1) 8.116(2) 61.43(2) 8.126(7) 8.118(2) 8.111 (3) 8.103(2) 8.095(2) 5.097(1) 5.096(1)
8.079(2) 17.672(2) 5.120(2) 8.137(2) 79.79(2) 17.76(2) 5.125(4) 17.68(1) 5.116(2) 17.66(2) 5.116(3) 26.52(3) 5.108(3) 26.53(1) 5.107(1) 8.092(2) 8.081(2)
V (/~3(f.u.)-~) Space group Type 181.02 183.57 184.63 184.82 183.53 183.19 182.95 182.82 182.09 181.77
P6/mmm Cmcm Pnma Cmcm Cmcm Cmcm Immm Immm P6/mmm P6/mmm
HfFe6Ge6 TbFe6Sn6 GdFe6Ge6 TbFe6Sn~ TbFe6Sn6 TbFe6Sn6 HoFe6Sn6 HoFe6Sn~ HfFe6Ge6 HfFe6Ge6
f.u., formula unit. c o m p o u n d s crystallize in the H f F e 6 G e 6 - t y p e structure, RFe6Ge6 (R--Tm, Er) in the HoFe6Sn6 s t r u c t u r e [3] a n d RFe6Ge6 (R----Y, Tb, Dy, Ho) in the TbFe6Sn6 s t r u c t u r e [3]. The case o f GdFe6Ge6 is m o r e c o m p l e x since it exhibits additional s u p e r s t r u c t u r e lines in its X-ray diffraction p a t t e r n (Fig. l ( a ) ) . W e t h e r e f o r e m a y p r o p o s e cell p a r a m e t e r s a n d a s p a c e g r o u p allowing the i n d e x a t i o n of t h e s u p e r s t r u c t u r e lines following the s c h e m e given in Fig. l ( b ) . A first splitting o f the HfFe6Ge6 s u p e r s t r u c t u r e lines into satellites, with a w a v e v e c t o r Q = ( 0 , q, 0) in the o r t h o h e x a g o n a l setting (a, 31/2a, c), yields the types e n c o u n t e r e d in the c o r r e s p o n d i n g s t a n n i d e s [3]. A s e c o n d splitting of these first satellites, with a w a v e v e c t o r I t = (r, 0, 0), leads to additional lines a r o u n d these. T h e Bragg angles o f the o b s e r v e d different lines lead to q = 0 . 8 6 7 and r = 0 . 5 5 5 and t h e n to the cell p a r a m e t e r s a ' = 12all, b ' = ell, C' 9 × 3'/2a~ with s p a c e g r o u p P n m a . T h e r e f i n e m e n t of the a t o m i c p o s i t i o n s is u n d e r way. =
3.2. M a g n e t i c p r o p e r t i e s 3.2.1. M a n g a n e s e c o m p o u n d s T h e t h e r m o m a g n e t i c p r o p e r t i e s o f t h e s e c o m p o u n d s are collected in Table 3. 3.2.1.1. P a r a m a g n e t i c state. T h e inverse susceptibility variation of s c a n d i u m , yttrium, n e o d y m i u m , s a m a r i u m , y t t e r b i u m a n d l u t e t i u m c o m p o u n d s o b e y s a C u r i e - W e i s s law. T h e p a r a m a g n e t i c Curie t e m p e r a t u r e s are r a t h e r high and characteristic of s t r o n g f e r r o m a g n e t i c couplings. T h e inverse susceptibility variation o f the o t h e r c o m p o u n d s d o e s n o t follow a C u r i e - W e i s s law, at least in the studied t e m p e r a t u r e range. It exhibits a c u r v a t u r e , s o m e t i m e s strong, w h o s e c o n c a v i t y is d i r e c t e d t o w a r d s the high t e m p e r a t u r e axis. This variation m a y be d e s c r i b e d with t h e h y p e r b o l i c f o r m characteristic of
102
,
•
,
42
•
i
44
46
•
,
-
i
48
-
50
2theta
(a)
Orthohexagonal
setting
. . . . . . . . . . . 7~::;zz. . . . ~\\-_-\\\5........... o 2-q
1
1
l-q 3
l+q 3
/ I I ",\ /
//
i i
// \\\
,
\
/ ~ /
o 2+q First splitting (OqO)
\
\ \\,
O+-r 2-q
1-r 1-q
l+r 1-q
1-r l+q
l+r l+q
3
3
3
3
3
h
L
i
I
O÷-r 2+q 3
i
h
Second splitting (rO0)
E
40 °
2*Theta
50 °
Co) Fig. 1. (a) Partial X-ray diffraction pattern of LuFeeGe6, DyFe6Ge6 and GdFe6Ge6. (b) Splitting scheme for the indexation transformation from hexagonal HfFesGe6-type structure to GdFesGe6 structure (for clarity the Miller indices are given vertically). ferrimagnetic
1
T+Op
X
C
compounds
~/ T-O
[9]
103 TABLE 3 Magnetic data for RMn~Ge6 (RmSc, Y, Nd, Sm, Gd-Lu) Compound
T s (K)
ScMn~Ge6 YMn~G% NdMn6Ge6 SmMn6GeB GdMn6Ge~ TbMn6Ge6 DyMn6Gee HoMn6Ge~ ErMn6Ge6 Tmin6Ge 6 YbMn~Ge8 LuMn6GeB
516 473 -
TC ~ -
417 441 463
-
427 423 466 475 482 480 509
-
-
-
0p (g)
~tLeff(~B)
~'LMn(~B) a
500 483 418 439 NCWb NCW NCW NCW NCW NCW 449 485
7.48 7.35 8.55 7.62
3.05 3.00 3.16 3.09
-
-
8.14 7.35
2.75 3.00
( ~~- - ~ ) ] ~ . bNCVq, non-Curie-Weiss.
"~M~ ( ~ B ) =
..I
1 / ~, ( g / e m u )
.+ .÷"
40
.+ + .+ ÷"
30 +" ~20
÷"
+
l0
200 '
~
I
400 I
I
600
L
800
T(K)
1 ~ _ _
Fig. 2. Temperature dependence of the inverse susceptibility of GdMn6Ge6: +, experimental; • • . , calculated. A t t e m p t s t o fit t h e p a r a m e t e r s g a v e u n s a t i s f a c t o r y r e s u l t s e x c e p t f o r G d M n 6 G e 8 w h i c h is t h e c o m p o u n d e x h i b i t i n g t h e l a r g e s t c u r v a t u r e . I n t h e other cases, owing to the shortness of the experimental temperature range and to the smaller curvatures, there are strong correlations between 0 and % yielding unrealistic Curie constants. Figure 2 gives the experimental and
104
calculated inverse susceptibility for GdMn6Ge6. The fitted parameters are: C = 1 4 . 8 2 (~e~=10.89 ]~B), 0p = 4 6 K, 0 = 3 0 2 K and T = 4 9 7 9 . The calculation of the paramagnetic manganese moment, assuming ]~Ga= 7.94 /~B, gives /~M,=3.04 ]~B which is in good agreement with the calculated values obtained with the Curie-Weiss-like compounds (Table 3). The low paramagnetic temperature 0p indicates strong additional anti-ferromagnetic interactions. 3.2.1.2. Ordering state. Three types of magnetic ordering are encountered, as follows. The lighter rare earth element compounds (neodymium, samarium, gadolinium) exhibit spontaneous magnetizations. The thermal variation of the susceptibility is typical of a P curve in the N~el classification for the ferrimagnetic compounds [10] (Fig. 3). The value of the magnetization which almost reaches saturation under an applied field of 1.5 T allows us to calculate the approximate manganese m o m e n t under the hypothesis of ferrimagnetic ordering: assuming a fully ordered rare earth moment at 90 K, the manganese m o m e n t s are close to 2 PB (Table 4). Hysteresis effects are observed for the neodymium and samarium compounds, the coercive fields remaining rather weak at 90 K (Table 4).
X (emu/g)
X
++% ÷+ + +÷+++ + +++++÷++++ + +++~÷,÷++++++++++++÷+++++++÷ +
30
(emu/g)
0.3
÷
i
20
F 0.2
+ 10
"
+ c ~. ~ • o • ,+,**o°,,,+ee,e
-~
~ ooooooo~ooooeooo o°~° + +,,+***,oo++,+++e*%~ee, +
r
1 O0
300
c
o°
~ O.l I
o% °°+o
i OOo
+++. . . . . ". °~°° . . . . '+
i
T(K)
SO0
Fig. 3. Thermal variation of the susceptibility: + , NdMnsGe8 ( H = 0.05 T, left-hand scale); O, LuMn~Ge6 ( H = 0 . 8 T, right-hand scale); O, LuFe6Ge~ ( H = 0 . 8 T, right-hand scale). TABLE 4 Magnetization values of RMneGe8 Cg--=Nd, Sin, Gd) at 300 and 90 K a Compound
a. (300 K) (~B)
vs (90 K) GaB)
Hc (kOe)
gJ (~B)
~M. (/~B)
NdMn6Ge8 SmMn6Ge6 GdMn6G%
8.9 9.1 4.8
8.1 11.4 4.6
0.6 0.3 0.0
3.28 0.72 7.00
2.01 2.02 1.93
a H = l . 5 T; calculated manganese m o m e n t values ~M~ (/~B)f~[crs (90 K)+gJ].
105
The paramagnetic rare earth (dysprosium to ytterbium) and diamagnetic rare earth elements (scandium, yttrium, lutetium) compounds are anti-ferromagnetic. All exhibit a rather sharp N6el point (Fig. 3) and a linear variation of the magnetization in the whole range of temperature and applied field. Down to 90 K there is no evidence of any ordering of the rare earth sublattice. The terbium compound has an intermediate behaviour since it presents a metamagnetic transition under fairly or very strong applied fields as a function of the temperature (Fig. 4).
3.2.2. Iron c o m p o u n d s The thermomagnetic properties of these compounds are summarized in Table 5. They are anti-ferromagnetic and their N6el points are strongly smoothed compared with the corresponding manganese compounds (Fig. 3). The inverse susceptibility curves do not exhibit a pronounced curvature as
o" ( o m ~ # g ) 11
20 il
,b
• Q
•
°
I
I!
O
I0
1
H(!) ©.U
1 ,0
Fig. 4. Magnetization curves for TbMn6G% at 300 K ( + ) , 323 K (E]), 373 K (O) and 423 K
(O). TABLE 5 Magnetic data for RF%G% CR---Sc, Y, Gd--Lu) Compound
TN (K)
0p (K)
/~e~ (/~B)
ScF%G% YF%G% GdF%G% TbFesG% DyF%G%
473 477 482 480 475
8.02 7.82 I 1.39 12.71 13.17
HoF%G%
473
ErF%G% TmF%G%
470 465
159 108 5 5 62 45 105 121
YbFe6G %
475
103
8.84
LuF%G%
452
112
7.65
13.34
12.07 10.74
106 in the manganese compounds and the paramagnetic Curie temperatures are much lower, involving stronger anti-ferromagnetic interactions. Down to 90 K there is no evidence of any ordering of the rare earth sublattice.
4. D i s c u s s i o n The magnetic properties of RT~Ge6 (T------Mn, Fe) exhibit a great variety of behaviours which can be discussed on the basis of our knowledge of related compounds and their magnetic structures. The binary compounds FeSn and FeGe B35 are built of ferromagnetic Fe(001) planes anti-ferromagnetically coupled to each other [11, 12]. Thus, assuming that in RFesGe6 compounds the iron sublattice keeps the same magnetic ordering, this implies a zero molecular field on the rare earth sublattice which will not favour its magnetic ordering. The binary compounds MnGe and MnSn do not occur but there are reports of magnetic structures of several RMn6Sn6 compounds [2, 13 ]. Within the diamagnetic rare earth compounds, which are not perturbed by the magnetism of the rare earth element, preliminary studies [13] show that YMn6Sn6 and ScMnBSn6 are complex helical structures built on ferromagnetic Mn(001) planes rotating over various angles along [001 ], and that LuMnsSn6 is collinear anti-ferromagnetic built on the same ferromagnetic planes with the following stacking sequence along c: + + - - + +--. Thus these compounds involve additional interplane ferromagnetic couplings which would explain the enhancement of the paramagnetic Curie temperature in the manganese compounds. Moreover, the occurrence of helical structures yields non-zero molecular fields on the rare earth sublattice which, through the polarization of the rare earth sublattice moments, will favour its magnetic ordering. There are nevertheless some differences between the magnetic behaviours of the RMnsGe6 and RMn6Sn6 compounds. First, as expected, the ordering temperature of the germanides with non-magnetic rare earth components are higher than those of the corresponding stannides, the ordering temperature decreasing with increasing size of the rare earth element. This behaviour comes from stronger Mn-Mn couplings due to shorter Mn-Mn interatomic distances. Another interesting difference is the evolution of the ordering temperature with the substitution of a magnetic rare earth element for a non-magnetic one: the substitution of gadolinium for lutetium enhances the ordering temperature of the stannides (TN(LU) -- 353 K, Tc(Gd) -- 435 K) whereas it acts in the opposite way for the germanides (TN(LU)= 509 K, Tc(Gd) = 463 K). This fact can be understood by considering an anti-ferromagnetic coupling between adjacent manganese planes surrounding the rare earth plane. In this case, both negative Mn-Mn and R-Mn interplane exchanges lead to a frustrated situation. Their relative strengths may explain the difference between the behaviours of the stannides and germanides. In the germanides, the shorter Mn-Mn interplane distances yield stronger Mn-Mn
107 c o u p l i n g s w h i c h are n o t fully c o m p e n s a t e d b y the R - M n c o u p l i n g s w h e n the f e r r i m a g n e t i c o r d e r t a k e s place. The result is a l o w e r n u m b e r o f ferrim a g n e t i c c o m p o u n d s in t h e g e r m a n i d e s . A last r e m a r k relates to t h e o c c u r r e n c e of f e r r i m a g n e t i c o r d e r in SmMn6Ge6 a n d NdMn6Ge6. It c o u l d c o m e f r o m s t r o n g S m ( N d ) - M n c o u p l i n g s b u t this effect d o e s n o t a p p e a r in the inverse susceptibility v a r i a t i o n s w h i c h closely follow C u r i e - W e i s s b e h a v i o u r . At this stage, we have to p o i n t out that s a m a r i u m a n d n e o d y m i u m are the l a r g e s t rare e a r t h e l e m e n t s in this series a n d that b o t h c o m p o u n d s are isotypic with YCo6Ge6. Thus their b e h a v i o u r m a y c o m e either f r o m the w e a k e n i n g o f the M n - M n interplane couplings due to larger M n - M n d i s t a n c e s o r f r o m a c h a n g e in t h e i n t e r p l a n e m a g n e t i c i n t e r a c t i o n s due to the f o r m a t i o n o f c o r r u g a t e d m a n g a n e s e planes in this d i s o r d e r e d structure.
5. C o n c l u s i o n
This s t u d y brings n e w i n f o r m a t i o n a n d q u e s t i o n s a b o u t the m a g n e t i c p r o p e r t i e s of R T e ~ c o m p o u n d s . F u r t h e r investigations will require low t e m p e r a t u r e m a g n e t i c m e a s u r e m e n t s and n e u t r o n diffraction studies. It w o u l d also be interesting t o look for o t h e r c o m p o u n d s YCo~Ge~-type s t r u c t u r e by alloying a p p r o p r i a t e elements. S u c h e x p e r i m e n t s are u n d e r way.
References
1 G. Venturini, B. Chafik el Idrissi and B. Malaman, J. Magn. Magn. Mater., g4 (1991) 349. 2 G. Venturini, B. Chafik el Idrissi, B. Malaman and D. Fruchart, J. Less-Common Met., 174 (1991) 143. 3 G. Venturini, B. Chafik el Idrissi and B. Malaman, Mater. Res. Bulk, 26 (1991) 1331. 4 O. E. Koretskaia and R. V. Skolozdra, Inorg. Mater. (USSR), Engl. Edn., 22 (1986) 606. 5 W. Bucholz and H. U. Schuster, Z. Anorg. AUg. Chem., 482 (1981) 40. 6 B. Chabot and E. Parth~, J. Less-Common Met., 96 (1983) L9. 7 R. R. Olenitch, L. G. Akselrud and Ya. P. Yarmoliuk, Dopov. Akad. Nauk. Ukr. RSR, Ser. A(2) (1981) 84. 8 G. Venturini, B. Chafik el Idrissi and B. Malaman, Mater. Res. BulL, 26 (1991) 431. 9 A. Herpin, Thdorie du Magndtisme, Biblioth~que des Sciences et Techniques Nucl~aires, Presses Universitaire de France, Paris, 1968. 10 L. N~el, Ann. Phys., 3 (1948) 137. 11 H. Watanabe and N. Kunitomi, J. Phys. Soc. Jpn., 21 (1966) 10, 1932. 12 K. Yamaguchi and H. Watanabe, J. Phys. Soc. Jpn., 22 (1967) 5, 1210. 13 G. Venturini et al., J. Alloys Comp., in the press.
99
Crystallographic data and magnetic properties of RT6Ge6 compounds (R-Sc, Y, Nd, Sm, Gd-Lu; T-Mn, Fe) G. Venturini, R. W e l t e r a n d B. M a l a m a n Labovatoire de C h i m i e du Solide Mindral, Associd a u CNRS No. 158, Universitd de Nancy, I, B.P. 239, 54506, Vandoeuvre les N a n c y Cedex (France)
(Received December 12, 1991)
Abstract The iron and manganese germanides RT6Ge6 (R-=Sc, Y, Nd, Sin, Gd-Lu; T=--Mn, Fe) have been investigated by X-ray diffraction and susceptibility measurements in the temperature range 90-900 K. They crystallize in the known HfFe6Ge6, YCo6G%, TbF%Sn6 or HoFe6Sn 6 type of structure except GdFe6Ge6 which is of a new type. All the compounds are anti-ferromagnetic except the RMn6Ge6 (RmNd, Sm, Gd) compounds which behave ferrimagnetically. Weak coercive fields (down to 1 kOe) are measured in RMn6G% (R~Nd, Sm).
1. I n t r o d u c t i o n P r e v i o u s s t u d i e s o n RMn6Sn8 c o m p o u n d s h a v e s h o w n the i n t e r e s t i n g m a g n e t i c p r o p e r t i e s o f this c l a s s o f m a t e r i a l [1, 2]. S o m e of t h e m exhibit large c o e r c i v e fields b u t the o r d e r i n g t e m p e r a t u r e of t h e s e f e r r i m a g n e t i c c o m p o u n d s r e m a i n s r a t h e r w e a k . In a n o t h e r way, a c r y s t a l l o g r a p h i c s t u d y o n RFe6Sn6 [3] c o m p o u n d s a l l o w e d u s t o p o i n t o u t the o c c u r r e n c e o f a l o n g - r a n g e o r d e r i n g in t h e s e c o m p o u n d s w h i c h w a s p r e v i o u s l y r e p o r t e d as i s o t y p i c of d i s o r d e r e d YCo~Ge6-type s t r u c t u r e [4]. Up to now, t h e c o r r e s p o n d i n g m a n g a n e s e a n d iron g e r m a n i d e s r e s p e c t i v e l y h a v e n o t b e e n or h a v e b e e n partially s t u d i e d [5, 6]. In o r d e r to e n h a n c e t h e t r a n s i t i o n t e m p e r a t u r e s , we h a v e i n v e s t i g a t e d t h e s e s y s t e m s .
2. E x p e r i m e n t a l d e t a i l s The compounds were synthesized from stoichiometric amounts of the e l e m e n t s . T h e m i x t u r e w a s c o m p a c t e d in a steel die a n d s e a l e d in a silica t u b e u n d e r a r g o n . T h e y w e r e t h e n h e a t e d f o r 2 w e e k s at 1 1 7 3 K for t h e iron c o m p o u n d s a n d 1073 K f o r t h e m a n g a n e s e c o m p o u n d s , e x c e p t f o r s a m a r i u m a n d n e o d y m i u m c o m p o u n d s w h i c h w e r e a n n e a l e d at l o w e r t e m p e r a t u r e ( 9 7 3 K). T h e p u r i t y o f t h e s a m p l e s w a s c h e c k e d b y X-ray analysis (Guinier Cu Ka, a n d c u r v e d d e t e c t o r I N E L CPS 120 Co Ker). T h e p a r a m e t e r s
0925-8388/92/$5.00
© 1992- Elsevier Sequoia. All rights reserved
100
were refined by a least-squares procedure. Magnetic measurements were carried out on a Faraday balance in the temperature range 8 0 - 8 5 0 K and under fields up to 1.5 T.
3. R e s u l t s
3.1. Crystallographic data 3.1.1. Manganese compounds Twelve new germanides have been characterized: E = S c , Y, Nd, Sm, Gd-Lu. The crystallographic features of these compounds are summarized in Table 1. Scandium, yttrium and the heavier rare earth element compounds crystallize in the HfFe~Ge~-type structure [7] while neodymium and samarium (i.e. the larger rare earth elements) compounds crystallize in the YCo6Ge6 structure. It is worth noting that the series of the manganese germanides is longer than that of manganese stannides since NdMnBGe6 is stable whereas NdMnsSn6 does not exist. Such a result is apparently surprising since generally the replacement of germanium with tin shifts the stability range of a given structural type towards the larger rare earth elements. This behaviour is probably due to close Sn-Sn contacts as we have previously remarked in the refinements of ScMn6Sn6 and TbMn6Sn6 [8]. According to the variation of the cell volumes, in YbMn6GeB, ytterbium seems to be in the III+ valence state.
3.1.2. Iron compounds Ten iron germanides have been characterized: R-=Sc, Y, Gd-Lu. The cell parameters, space groups and structure type of these compounds are summarized in Table 2. The YbFesGe6 cell does not exhibit any volume anomaly. The smaller rare earth elements (scandium, ytterbium, lutetium) TABLE 1 Cell parameters and structure types of RMn6Ge8 (R------Sc,Y, Nd, Sm, Gd-Lu) a Compound
a (.~)
c (~)
V (/~3)
Type
ScMn6Ge8 YMI~Ge6 NdMn6Ge8 SmMn6Ge~ GdMn6Ge 6 TbMn6Ge6 DyMn6Ge6 HoMn~Ge8 ErMn~Ge8 TmMn~Ge6 YbMn~Ge6 LuMnsGe8
5.177(6) 5.233(2) 5.258(1) 5.245(2) 5.242(2) 5.232(1) 5.228(1) 5.230(1) 5.221 (2) 5.219(1) 5.212(1) 5.211 (1)
8.084(4) 8.153(3) 8.208(1) 8.187(2) 8.184(3) 8.169(2) 8.163(2) 8.155(2) 8.142(3) 8.139(1) 8.132(1) 8.134(3)
187.6 193.4 196.5 195.0 194.8 193.7 193.2 193.2 192.2 191.9 191.3 191.3
HfFesGe8 HfFe6Ge8 YCo6Ge8 YCosGe8 HfFe6Ge6 HfFesGe8 HfFesGe8 HfFesGe8 HfFe~Ge6 HfFe~Ge8 HfFesGe8 HfFe6Ges
"Space group, P 6 / m m m .
101 TABLE 2 Cell parameters and structure types of RFe6Ge6 (R-=Sc, Y, Gd-Lu) Compound
a (/~)
b (/~)
c (/~)
ScF%G% YF%Ge6 GdFe6Ge6 TbFe6Ge6 DyF%Ge~ HoFe6Ge6 ErF%G% TmFe6G% YbFe6Ge6 LuFe6G%
5.087(1) 8.116(2) 61.43(2) 8.126(7) 8.118(2) 8.111 (3) 8.103(2) 8.095(2) 5.097(1) 5.096(1)
8.079(2) 17.672(2) 5.120(2) 8.137(2) 79.79(2) 17.76(2) 5.125(4) 17.68(1) 5.116(2) 17.66(2) 5.116(3) 26.52(3) 5.108(3) 26.53(1) 5.107(1) 8.092(2) 8.081(2)
V (/~3(f.u.)-~) Space group Type 181.02 183.57 184.63 184.82 183.53 183.19 182.95 182.82 182.09 181.77
P6/mmm Cmcm Pnma Cmcm Cmcm Cmcm Immm Immm P6/mmm P6/mmm
HfFe6Ge6 TbFe6Sn6 GdFe6Ge6 TbFe6Sn~ TbFe6Sn6 TbFe6Sn6 HoFe6Sn6 HoFe6Sn~ HfFe6Ge6 HfFe6Ge6
f.u., formula unit. c o m p o u n d s crystallize in the H f F e 6 G e 6 - t y p e structure, RFe6Ge6 (R--Tm, Er) in the HoFe6Sn6 s t r u c t u r e [3] a n d RFe6Ge6 (R----Y, Tb, Dy, Ho) in the TbFe6Sn6 s t r u c t u r e [3]. The case o f GdFe6Ge6 is m o r e c o m p l e x since it exhibits additional s u p e r s t r u c t u r e lines in its X-ray diffraction p a t t e r n (Fig. l ( a ) ) . W e t h e r e f o r e m a y p r o p o s e cell p a r a m e t e r s a n d a s p a c e g r o u p allowing the i n d e x a t i o n of t h e s u p e r s t r u c t u r e lines following the s c h e m e given in Fig. l ( b ) . A first splitting o f the HfFe6Ge6 s u p e r s t r u c t u r e lines into satellites, with a w a v e v e c t o r Q = ( 0 , q, 0) in the o r t h o h e x a g o n a l setting (a, 31/2a, c), yields the types e n c o u n t e r e d in the c o r r e s p o n d i n g s t a n n i d e s [3]. A s e c o n d splitting of these first satellites, with a w a v e v e c t o r I t = (r, 0, 0), leads to additional lines a r o u n d these. T h e Bragg angles o f the o b s e r v e d different lines lead to q = 0 . 8 6 7 and r = 0 . 5 5 5 and t h e n to the cell p a r a m e t e r s a ' = 12all, b ' = ell, C' 9 × 3'/2a~ with s p a c e g r o u p P n m a . T h e r e f i n e m e n t of the a t o m i c p o s i t i o n s is u n d e r way. =
3.2. M a g n e t i c p r o p e r t i e s 3.2.1. M a n g a n e s e c o m p o u n d s T h e t h e r m o m a g n e t i c p r o p e r t i e s o f t h e s e c o m p o u n d s are collected in Table 3. 3.2.1.1. P a r a m a g n e t i c state. T h e inverse susceptibility variation of s c a n d i u m , yttrium, n e o d y m i u m , s a m a r i u m , y t t e r b i u m a n d l u t e t i u m c o m p o u n d s o b e y s a C u r i e - W e i s s law. T h e p a r a m a g n e t i c Curie t e m p e r a t u r e s are r a t h e r high and characteristic of s t r o n g f e r r o m a g n e t i c couplings. T h e inverse susceptibility variation o f the o t h e r c o m p o u n d s d o e s n o t follow a C u r i e - W e i s s law, at least in the studied t e m p e r a t u r e range. It exhibits a c u r v a t u r e , s o m e t i m e s strong, w h o s e c o n c a v i t y is d i r e c t e d t o w a r d s the high t e m p e r a t u r e axis. This variation m a y be d e s c r i b e d with t h e h y p e r b o l i c f o r m characteristic of
102
,
•
,
42
•
i
44
46
•
,
-
i
48
-
50
2theta
(a)
Orthohexagonal
setting
. . . . . . . . . . . 7~::;zz. . . . ~\\-_-\\\5........... o 2-q
1
1
l-q 3
l+q 3
/ I I ",\ /
//
i i
// \\\
,
\
/ ~ /
o 2+q First splitting (OqO)
\
\ \\,
O+-r 2-q
1-r 1-q
l+r 1-q
1-r l+q
l+r l+q
3
3
3
3
3
h
L
i
I
O÷-r 2+q 3
i
h
Second splitting (rO0)
E
40 °
2*Theta
50 °
Co) Fig. 1. (a) Partial X-ray diffraction pattern of LuFeeGe6, DyFe6Ge6 and GdFe6Ge6. (b) Splitting scheme for the indexation transformation from hexagonal HfFesGe6-type structure to GdFesGe6 structure (for clarity the Miller indices are given vertically). ferrimagnetic
1
T+Op
X
C
compounds
~/ T-O
[9]
103 TABLE 3 Magnetic data for RMn~Ge6 (RmSc, Y, Nd, Sm, Gd-Lu) Compound
T s (K)
ScMn~Ge6 YMn~G% NdMn6Ge6 SmMn6GeB GdMn6Ge~ TbMn6Ge6 DyMn6Gee HoMn6Ge~ ErMn6Ge6 Tmin6Ge 6 YbMn~Ge8 LuMn6GeB
516 473 -
TC ~ -
417 441 463
-
427 423 466 475 482 480 509
-
-
-
0p (g)
~tLeff(~B)
~'LMn(~B) a
500 483 418 439 NCWb NCW NCW NCW NCW NCW 449 485
7.48 7.35 8.55 7.62
3.05 3.00 3.16 3.09
-
-
8.14 7.35
2.75 3.00
( ~~- - ~ ) ] ~ . bNCVq, non-Curie-Weiss.
"~M~ ( ~ B ) =
..I
1 / ~, ( g / e m u )
.+ .÷"
40
.+ + .+ ÷"
30 +" ~20
÷"
+
l0
200 '
~
I
400 I
I
600
L
800
T(K)
1 ~ _ _
Fig. 2. Temperature dependence of the inverse susceptibility of GdMn6Ge6: +, experimental; • • . , calculated. A t t e m p t s t o fit t h e p a r a m e t e r s g a v e u n s a t i s f a c t o r y r e s u l t s e x c e p t f o r G d M n 6 G e 8 w h i c h is t h e c o m p o u n d e x h i b i t i n g t h e l a r g e s t c u r v a t u r e . I n t h e other cases, owing to the shortness of the experimental temperature range and to the smaller curvatures, there are strong correlations between 0 and % yielding unrealistic Curie constants. Figure 2 gives the experimental and
104
calculated inverse susceptibility for GdMn6Ge6. The fitted parameters are: C = 1 4 . 8 2 (~e~=10.89 ]~B), 0p = 4 6 K, 0 = 3 0 2 K and T = 4 9 7 9 . The calculation of the paramagnetic manganese moment, assuming ]~Ga= 7.94 /~B, gives /~M,=3.04 ]~B which is in good agreement with the calculated values obtained with the Curie-Weiss-like compounds (Table 3). The low paramagnetic temperature 0p indicates strong additional anti-ferromagnetic interactions. 3.2.1.2. Ordering state. Three types of magnetic ordering are encountered, as follows. The lighter rare earth element compounds (neodymium, samarium, gadolinium) exhibit spontaneous magnetizations. The thermal variation of the susceptibility is typical of a P curve in the N~el classification for the ferrimagnetic compounds [10] (Fig. 3). The value of the magnetization which almost reaches saturation under an applied field of 1.5 T allows us to calculate the approximate manganese m o m e n t under the hypothesis of ferrimagnetic ordering: assuming a fully ordered rare earth moment at 90 K, the manganese m o m e n t s are close to 2 PB (Table 4). Hysteresis effects are observed for the neodymium and samarium compounds, the coercive fields remaining rather weak at 90 K (Table 4).
X (emu/g)
X
++% ÷+ + +÷+++ + +++++÷++++ + +++~÷,÷++++++++++++÷+++++++÷ +
30
(emu/g)
0.3
÷
i
20
F 0.2
+ 10
"
+ c ~. ~ • o • ,+,**o°,,,+ee,e
-~
~ ooooooo~ooooeooo o°~° + +,,+***,oo++,+++e*%~ee, +
r
1 O0
300
c
o°
~ O.l I
o% °°+o
i OOo
+++. . . . . ". °~°° . . . . '+
i
T(K)
SO0
Fig. 3. Thermal variation of the susceptibility: + , NdMnsGe8 ( H = 0.05 T, left-hand scale); O, LuMn~Ge6 ( H = 0 . 8 T, right-hand scale); O, LuFe6Ge~ ( H = 0 . 8 T, right-hand scale). TABLE 4 Magnetization values of RMneGe8 Cg--=Nd, Sin, Gd) at 300 and 90 K a Compound
a. (300 K) (~B)
vs (90 K) GaB)
Hc (kOe)
gJ (~B)
~M. (/~B)
NdMn6Ge8 SmMn6Ge6 GdMn6G%
8.9 9.1 4.8
8.1 11.4 4.6
0.6 0.3 0.0
3.28 0.72 7.00
2.01 2.02 1.93
a H = l . 5 T; calculated manganese m o m e n t values ~M~ (/~B)f~[crs (90 K)+gJ].
105
The paramagnetic rare earth (dysprosium to ytterbium) and diamagnetic rare earth elements (scandium, yttrium, lutetium) compounds are anti-ferromagnetic. All exhibit a rather sharp N6el point (Fig. 3) and a linear variation of the magnetization in the whole range of temperature and applied field. Down to 90 K there is no evidence of any ordering of the rare earth sublattice. The terbium compound has an intermediate behaviour since it presents a metamagnetic transition under fairly or very strong applied fields as a function of the temperature (Fig. 4).
3.2.2. Iron c o m p o u n d s The thermomagnetic properties of these compounds are summarized in Table 5. They are anti-ferromagnetic and their N6el points are strongly smoothed compared with the corresponding manganese compounds (Fig. 3). The inverse susceptibility curves do not exhibit a pronounced curvature as
o" ( o m ~ # g ) 11
20 il
,b
• Q
•
°
I
I!
O
I0
1
H(!) ©.U
1 ,0
Fig. 4. Magnetization curves for TbMn6G% at 300 K ( + ) , 323 K (E]), 373 K (O) and 423 K
(O). TABLE 5 Magnetic data for RF%G% CR---Sc, Y, Gd--Lu) Compound
TN (K)
0p (K)
/~e~ (/~B)
ScF%G% YF%G% GdF%G% TbFesG% DyF%G%
473 477 482 480 475
8.02 7.82 I 1.39 12.71 13.17
HoF%G%
473
ErF%G% TmF%G%
470 465
159 108 5 5 62 45 105 121
YbFe6G %
475
103
8.84
LuF%G%
452
112
7.65
13.34
12.07 10.74
106 in the manganese compounds and the paramagnetic Curie temperatures are much lower, involving stronger anti-ferromagnetic interactions. Down to 90 K there is no evidence of any ordering of the rare earth sublattice.
4. D i s c u s s i o n The magnetic properties of RT~Ge6 (T------Mn, Fe) exhibit a great variety of behaviours which can be discussed on the basis of our knowledge of related compounds and their magnetic structures. The binary compounds FeSn and FeGe B35 are built of ferromagnetic Fe(001) planes anti-ferromagnetically coupled to each other [11, 12]. Thus, assuming that in RFesGe6 compounds the iron sublattice keeps the same magnetic ordering, this implies a zero molecular field on the rare earth sublattice which will not favour its magnetic ordering. The binary compounds MnGe and MnSn do not occur but there are reports of magnetic structures of several RMn6Sn6 compounds [2, 13 ]. Within the diamagnetic rare earth compounds, which are not perturbed by the magnetism of the rare earth element, preliminary studies [13] show that YMn6Sn6 and ScMnBSn6 are complex helical structures built on ferromagnetic Mn(001) planes rotating over various angles along [001 ], and that LuMnsSn6 is collinear anti-ferromagnetic built on the same ferromagnetic planes with the following stacking sequence along c: + + - - + +--. Thus these compounds involve additional interplane ferromagnetic couplings which would explain the enhancement of the paramagnetic Curie temperature in the manganese compounds. Moreover, the occurrence of helical structures yields non-zero molecular fields on the rare earth sublattice which, through the polarization of the rare earth sublattice moments, will favour its magnetic ordering. There are nevertheless some differences between the magnetic behaviours of the RMnsGe6 and RMn6Sn6 compounds. First, as expected, the ordering temperature of the germanides with non-magnetic rare earth components are higher than those of the corresponding stannides, the ordering temperature decreasing with increasing size of the rare earth element. This behaviour comes from stronger Mn-Mn couplings due to shorter Mn-Mn interatomic distances. Another interesting difference is the evolution of the ordering temperature with the substitution of a magnetic rare earth element for a non-magnetic one: the substitution of gadolinium for lutetium enhances the ordering temperature of the stannides (TN(LU) -- 353 K, Tc(Gd) -- 435 K) whereas it acts in the opposite way for the germanides (TN(LU)= 509 K, Tc(Gd) = 463 K). This fact can be understood by considering an anti-ferromagnetic coupling between adjacent manganese planes surrounding the rare earth plane. In this case, both negative Mn-Mn and R-Mn interplane exchanges lead to a frustrated situation. Their relative strengths may explain the difference between the behaviours of the stannides and germanides. In the germanides, the shorter Mn-Mn interplane distances yield stronger Mn-Mn
107 c o u p l i n g s w h i c h are n o t fully c o m p e n s a t e d b y the R - M n c o u p l i n g s w h e n the f e r r i m a g n e t i c o r d e r t a k e s place. The result is a l o w e r n u m b e r o f ferrim a g n e t i c c o m p o u n d s in t h e g e r m a n i d e s . A last r e m a r k relates to t h e o c c u r r e n c e of f e r r i m a g n e t i c o r d e r in SmMn6Ge6 a n d NdMn6Ge6. It c o u l d c o m e f r o m s t r o n g S m ( N d ) - M n c o u p l i n g s b u t this effect d o e s n o t a p p e a r in the inverse susceptibility v a r i a t i o n s w h i c h closely follow C u r i e - W e i s s b e h a v i o u r . At this stage, we have to p o i n t out that s a m a r i u m a n d n e o d y m i u m are the l a r g e s t rare e a r t h e l e m e n t s in this series a n d that b o t h c o m p o u n d s are isotypic with YCo6Ge6. Thus their b e h a v i o u r m a y c o m e either f r o m the w e a k e n i n g o f the M n - M n interplane couplings due to larger M n - M n d i s t a n c e s o r f r o m a c h a n g e in t h e i n t e r p l a n e m a g n e t i c i n t e r a c t i o n s due to the f o r m a t i o n o f c o r r u g a t e d m a n g a n e s e planes in this d i s o r d e r e d structure.
5. C o n c l u s i o n
This s t u d y brings n e w i n f o r m a t i o n a n d q u e s t i o n s a b o u t the m a g n e t i c p r o p e r t i e s of R T e ~ c o m p o u n d s . F u r t h e r investigations will require low t e m p e r a t u r e m a g n e t i c m e a s u r e m e n t s and n e u t r o n diffraction studies. It w o u l d also be interesting t o look for o t h e r c o m p o u n d s YCo~Ge~-type s t r u c t u r e by alloying a p p r o p r i a t e elements. S u c h e x p e r i m e n t s are u n d e r way.
References
1 G. Venturini, B. Chafik el Idrissi and B. Malaman, J. Magn. Magn. Mater., g4 (1991) 349. 2 G. Venturini, B. Chafik el Idrissi, B. Malaman and D. Fruchart, J. Less-Common Met., 174 (1991) 143. 3 G. Venturini, B. Chafik el Idrissi and B. Malaman, Mater. Res. Bulk, 26 (1991) 1331. 4 O. E. Koretskaia and R. V. Skolozdra, Inorg. Mater. (USSR), Engl. Edn., 22 (1986) 606. 5 W. Bucholz and H. U. Schuster, Z. Anorg. AUg. Chem., 482 (1981) 40. 6 B. Chabot and E. Parth~, J. Less-Common Met., 96 (1983) L9. 7 R. R. Olenitch, L. G. Akselrud and Ya. P. Yarmoliuk, Dopov. Akad. Nauk. Ukr. RSR, Ser. A(2) (1981) 84. 8 G. Venturini, B. Chafik el Idrissi and B. Malaman, Mater. Res. BulL, 26 (1991) 431. 9 A. Herpin, Thdorie du Magndtisme, Biblioth~que des Sciences et Techniques Nucl~aires, Presses Universitaire de France, Paris, 1968. 10 L. N~el, Ann. Phys., 3 (1948) 137. 11 H. Watanabe and N. Kunitomi, J. Phys. Soc. Jpn., 21 (1966) 10, 1932. 12 K. Yamaguchi and H. Watanabe, J. Phys. Soc. Jpn., 22 (1967) 5, 1210. 13 G. Venturini et al., J. Alloys Comp., in the press.